1. Field of the Invention
The present invention relates to a thermal-type flow sensor and a flow rate detecting element therefor which includes heat generating members for measuring a flowing speed or a flow rate of a fluid on the basis of heat transfer from heat generating members or parts heated by the heat generating members to the fluid. Such thermal-type flow sensor can find application to measurement of an intake airflow in an internal combustion engine, for example.
2. Description of Related Art
For having better understanding of the invention, background techniques thereof will first be reviewed. FIG. 13 is a view showing a flow rate detecting element employed in a thermal-type flow sensor known heretofore, as disclosed in Japanese Unexamined Patent Application Publication No. 185416/1989 (JP-A-1-185416), and FIG. 14 is a circuit diagram showing a circuit configuration of a thermal-type flow sensor known heretofore. Referring to FIG. 13, the flow rate detecting element employed in the hitherto known or conventional thermal-type flow sensor is constituted by a planar substrate 38 and heat generating resistors 39a and 39b each formed of platinum in a thin film deposited on a surface of the planar substrate 38 through sputtering and photoetching process. The heat generating resistor 39a is destined to be disposed at an upstream position as viewed in the flow direction of a fluid of concern (hereinafter this heat generating resistor 39a will also be referred to also as the upstream heat generating resistor), while the heat generating resistor 39b is disposed at a downstream position as viewed in the flow direction (this heat generating resistor 39b will also be referred to also as the downstream heat generating resistor). The upstream heat generating resistor 39a and the downstream heat generating resistors 39b have respective outer surfaces each coated with a thin film of alumina or silicon oxide. The upstream heat generating resistor 39a and the downstream heat generating resistor 39b are connected to terminals 40a and 40b for external connection, respectively, which are provided on the surface of the planar substrate 38.
Further provided are temperature compensating resistors (not shown) for detecting fluid temperature, each of which is implemented as a heat-sensitive resistor in the form of a platinum thin film through a process similar to that for forming the heat generating resistors mentioned above. The temperature compensating resistor is so designed as to exhibit at least five hundred times as high a resistance value as the heat generating resistor.
Now, referring to FIG. 14, it can be seen that the upstream heat generating resistor 39a, the downstream heat generating resistor 39b and the temperature compensating resistors 41a and 41b are disposed within a main passage 19 through which the fluid flows, the flow rate of which is to be measured. Such fluid may be intake air in an internal combustion engine and hence will be also referred to as the intake air or simply as the air.
The upstream heat generating resistor 39aconstitutes a bridge circuit together with the upstream temperature compensating resistor 41a disposed upstream and fixed resistors 42a, 43a and 44a, wherein the potential making appearance at a neutral point between the fixed resistors 42a and 44a and the potential at a neutral point between the fixed resistor 43a and the upstream heat generating resistor 39a are applied, respectively, to the input terminals of a differential amplifier 45a for detecting a potential difference.
A potential difference signal outputted from the differential amplifier 45a is applied to a base of a transistor 46a. In this manner, there is realized a closed lop control for equalizing the potentials at the neutral points mentioned above. Parenthetically, emitter electrode of the transistor 46a is connected to a junction between the upstream temperature compensating resistor 41a and the upstream heat generating resistor 39a while the collector thereof is connected to a power supply.
Similarly, the downstream heat generating resistor 39b constitutes another bridge circuit together with the downstream temperature compensating resistor 41b disposed downstream and fixed resistors 42b, 43b and 44b, wherein the bridge circuit constitutes a closed loop control circuit in cooperation with a differential amplifier 45b and a transistor 46b. More specifically, potential making appearance at a neutral point between the fixed resistors 42b and 44b and potential at a neutral point between the fixed resistor 43b and the downstream heat generating resistor 39b are applied, respectively, to the input terminals of the differential amplifier 45b for detecting a potential difference.
Resistance values of the fixed resistors constituting the bridge circuits are so designed that the temperatures of the heat generating resistors 39a and 39b are higher by ca. 100.degree. C. than the intake air temperature detected by the temperature compensating resistors 41a and 41b, respectively. At this juncture, it should be mentioned that the resistance value RH of the upstream heat generating resistor 39a, for example, can be given by the undermentioned expression on the basis of bridge balancing conditions. EQU RH={(Rk+R1).multidot.R3}/R2
where Rk represents a resistance value of the upstream temperature compensating resistor 41a,
R1 represents a resistance value of the fixed resistor 42a, PA1 R2 represents a resistance value of the fixed resistor 44a, and PA1 R3 represents a resistance value of the fixed resistor 43a.
As mentioned previously, the resistance value of the upstream heat generating resistor 39a is so set that the temperature thereof becomes higher by ca. 100.degree. C. than the intake air temperature. Thus, the resistance value of the upstream temperature compensating resistor 41a as well as that of the upstream heat generating resistor 39a remains constant so long as the intake air temperature is constant. The current flowing through the bridge circuit is so controlled through cooperation of the differential amplifier 45a and the transistor 46a that the resistance of the upstream heat generating resistor 39a assumes a constant value independent of the flow rate of the intake air. In this manner, the current flowing through the upstream heat generating resistor 39a can be detected as a voltage drop making appearance across the fixed resistor 43a, whereby the air flow rate can be determined on the basis of the voltage drop mentioned above.
In the conventional thermal-type flow sensor described above, the quantity of heat transferred to the air flow increases as the flow rate thereof increases. On the other hand, because the air flowing along the downstream heat generating resistor 39b has already been heated by the upstream heat generating resistor 39a, the rate of heat transfer to the air flow from the downstream heat generating resistor 39b is low when compared with the heat transfer from the upstream heat generating resistor 39a to the air flow. To say in another way, the upstream heat generating resistor 39a is cooled at higher rate than the downstream heat generating resistor 39b, wherein difference in the rate of cooling between the upstream heat generating resistor 39a and the downstream heat generating resistor 39b increases as a function of the air flow rate.
Consequently, the current required for heating the upstream heat generating resistor 39a in order to maintain constant the resistance value thereof is larger than the current required for heating the downstream heat generating resistor 39b to hold constant the resistance value thereof, wherein the difference in the heating current between the upstream heat generating resistor 39a and the downstream heat generating resistor 39b increases as the flow rate of the air increases. Thus, difference between the inter-terminal voltage Va making appearance across the fixed resistor 43a and the inter-terminal voltage Vb across the fixed resistor 43b bears a functional relation to the air-flow rate or the quantity of air flowing through a passage having a predetermined sectional area, because the heating current is equivalent to the heat transfer rate while the heat transfer rate is given as a function of the air flow rate, as mentioned above.
Thus, the inter-terminal voltage Va becomes higher than the inter-terminal voltage Vb (i.e., Va&gt;Vb) when the air flows in the forward direction (i.e., from the upstream side to the downstream side), whereas the inter-terminal voltage Vb becomes higher than the inter-terminal voltage Va (i.e., Vb&gt;Va) when the air flows in the reverse direction. Accordingly, the difference between the inter-terminal voltages Va and Vb can be utilized as a vector quantity representing both the absolute value of the rate of air flow and the direction thereof. Thus, there can be realized the thermal-type flow sensor which is capable of detecting the direction of the air flow on the basis of difference between the inter-terminal voltages Va and Vb, which difference can be determined by a differential amplifier 47.
In the conventional thermal-type flow sensor described above, the resistors used for constituting the bridge circuits including the upstream heat generating resistor 39a and the downstream heat generating resistor 39b, respectively, are identical in respect to the resistance values. As a consequence, when the difference between the inter-terminal voltages Va and Vb (i.e., Va-Vb) is made use of for deriving the flow rate signal outputted from the thermal-type flow sensor, the difference becomes zero (i.e., Va-Vb=0) when the flow rate is zero while the difference assumes a value of opposite or negative polarity (i.e., Va-Vb&lt;0) when the air flows reversely from the downstream side toward the upstream side.
In case the thermal-type flow sensor of the output characteristics described above is used in association with an apparatus or system to which the output signal of the thermal-type flow sensor is inputted as in the case of a fuel control unit for an internal combustion engine of a motor vehicle, the circuit configuration is so implemented as to be capable of identifying the minus or negative polarity of the input signal in addition to the magnitude thereof, which involves complication of the circuit configuration.
Of course, there may be used a signal resulting from addition of a predetermined bias voltage Vob to the difference between the inter-terminal voltages Va and Vb (i.e., Va-Vb+Vob) internally of the thermal-type flow sensor. However, in that case, a bias voltage adding circuit has to be additionally provided internally of the thermal-type flow sensor. Furthermore, in case the thermal-type flow sensor is destined for use as the intake air fixed resistor sensor in an internal combustion engine system for a motor vehicle, the bias voltage adding circuit mentioned above is required to be so designed that fluctuation of the predetermined bias voltage Vob due to change in the ambient temperature can be suppressed to a possible minimum in view of the fact that the ambient temperature varies over a range of -30.degree. C. to 110.degree. C.
Thus, the measures described above will be accompanied with correspondingly increased expenditure.
On the other hand, when the thermal-type flow sensor is employed in association with the fuel control for an internal combustion engine of a motor vehicle, the flow rate signal may assume a pulsating waveform containing reverse flow rate components in the operating a range in which the throttle valve is opened with such a large opening degree that valve-overlap may take place. In general, however, the flow rate is considerably lower than that in the forward direction. Accordingly, it is preferred to design the thermal-type flow sensor employed in the application mentioned above such that it can exhibit high sensitivity in the forward flow rate measurement.
However, in the conventional thermal-type flow sensor described above, the sensitivity thereof is essentially same for the air flow in both the forward direction and the reverse direction, which means that limitation is imposed on the measurement of the maximum flow rate in the forward direction.